# Generating a circuit

The Build Circuit button will construct a circuit whose gates correspond to the currently chosen expressions for each output. The circuit's inputs and outputs will be displayed in top-down order corresponding to how they appear under the Inputs and Outputs tabs. Generally speaking, the constructed circuit will be attractive; and, indeed, one application of Logisim's Combinational Analysis module is to beautify poorly drawn circuits. Still, as with any automatic formatting, it will not express the structural details that a human-drawn circuit would.

When you click the Build Circuit button, a dialog box will appear prompting you to choose which project where you want the circuit and the name you wish to give it.

If you type the name of an existing circuit, then that circuit will be replaced (after Logisim prompts you to confirm that you really want to do this).

The Build Circuit dialog includes two options.

- The Use Two-Input Gates Only option specifies that you want all gates constructed to have two inputs. (NOT gates, of course, constitute an exception to this rule.)
- TheUse NAND Gates Only option specifies that you would like it to translate the circuit into one using only NAND gates. You can select both options if you want to use only two-input NAND gates.

Logisim cannot construct a NAND-only circuit for an expression containing any XOR operators. This option will therefore be disabled if any outputs' expressions contain XORs.

Next: User's Guide.